Hydraulic elevators are comprised of a cab, a plunger attached to the cab either directly or by means of a roping configuration, and a cylinder housing the plunger. Hydraulic fluid is impelled into the cylinder to drive the plunger and the cab attached thereto, upwardly. The fluid is typically impelled into the cylinder by means of a constant speed motor which drives the fixed displacement pump.
This type of hydraulic elevator requires valves to control the cab speed during acceleration, leveling, and lowering of the elevator. The valves waste energy while controlling the motion of the elevator by discharging excess fluid flow. Valves may also be noisy and provide relatively poor control.
Some hydraulic elevators utilize a variable speed pump and motor. In this type of elevator, hydraulic fluid is impelled into and out of the cylinder to drive the plunger and the cab attached thereto upwardly and downwardly. The variable speed pump and motor are known to reduce the control problem and greatly simplify the number of valves. However, a valve is still required to maintain the elevator at a landing. One such type of a valve is disclosed in U.S. patent application 07/701,369, wherein the valve controls the pressure between the pump and the elevator, thereby controlling the movement of the elevator car.
In case of excessive pressure build up, most hydraulic elevator systems require some means of limiting pressure within the system to preserve the integrity thereof. Therefore, most systems provide some type of a switch or a relief valve that in case of excessive pressure increase within the system, limits such pressure build up. However, safety problems arise when the excess pressure is limited.